This invention relates to an improvement in a system for mounting an engine to the structure of an aircraft. The invention herein described was made in the course of or under a contract, or a subcontract thereunder, with the United States Department of the Air Force.
In the aircraft industry, it is generally desirable that aircraft engines be readily adaptable to a variety of aircraft without substantial modification or redesign. In many instances, mounting of an aircraft engine, designed for one type of aircraft, to another aircraft of a different type can result in additional stress and deflection in the structural components of the engine. Additional stress and deflection are typically encountered where an engine originally designed to carry its own exhaust nozzle is mounted to an aircraft having the exhaust nozzle as an integral part of the aircraft structure.
Aircraft engines carrying the exhaust nozzle generally are subject to lower net reactive thrust forces than aircraft engines used with an exhaust nozzle integrally attached to the aircraft. In the former instance, lower net reactive thrust forces result from the transfer of additional nozzle loads, which oppose the reactive thrust forces, directly to the engine. In the latter instance the nozzle loads are transferred to the aircraft and hence the net reactive thrust forces on the engine are of higher magnitude by an amount equal to the nozzle load.
In aircraft mounted nozzle systems the higher magnitude reactive thrust forces on the engine are reacted by the engine main thrust mount which is usually offset from the center of thrust of the engine thereby resulting in the imposition of additional bending moments and associated bending stresses and deflections on the engine. These additional bending stresses and deflections are usually not present in engines carrying the exhaust nozzle since in that instance the nozzle loads are transferred to the engine uniformly around the circumference of the engine thereby reducing the net reactive thrust load on the engine.
The aforementioned additional bending stresses and deflections in appropriate circumstances can cause rubbing between rotating portions of the engine compressor and turbine and their respective stationary elements at high power settings. This results in excessive operating clearances at lower power settings and an associated decrease in engine efficiency.
The deleterious effects of the additional bending stresses and deflections described above can, of course, be reduced by increasing the thickness of the load bearing elements of the engine or by fabricating the elements from stronger materials. However, these approaches are not compatible with overall objectives of having an engine which is readily adaptable with minimum design modification and with a combination of minimum weight and minimum cost.
While the use of auxiliary multiple rigid mounts may also eliminate the aforementioned deflections, such an approach does not allow for thermal growth of the engine under operating conditions and requires close tolerance control of the interface between the engine and the mount. Furthermore, it is desirable to eliminate only those bending stresses and deflections associated with the increase in the reactive thrust load as described above. Generally, aircraft engines are mounted in a manner permitting a limited amount of relative movement between the engine or parts thereof and surrounding aircraft structure when the engine is generating thrust. In order to accommodate the relative movement, the engine itself is designed to flex in a specific fashion and undergo acceptable limited stress and deflection. The use of auxiliary fixed rigid mounts which eliminate not only the aforementioned additional stress and deflection but also preclude the relative movement between the engine and the aircraft, alters the normal compatibility between the engine and the aircraft, constrains the engine from flexing and induces internal loads on the engine inconsistent with its original design. Accordingly, auxiliary fixed mounts do not provide an acceptable solution to the aforementioned problems.